The Using MOSFETS Website

H-bridge using P and N channel FETs

H-bridge using P and N channel FETs

Theory of Operation

by Eugene Blanchard

This H-bridge uses MOSFETs for one main reason - to improve the efficiency
of the bridge. When BJT transistors (normal transistors) were used, they had a
saturation voltage of approximately 1V across the collector emitter junction
when turned on. My power supply was 10V and I was consuming 2V across the two
transistor required to control the direction of the motor. 20% of my power was
eaten up by the transistors. I tried darlingtons etc... nothing worked. The
transistors also would get quite hot - no room for heatsinks.

MOSFET advantage

I chose MOSFETs because when they turn on they have an ON resistance called
RDS(on). This is the resistance between the Drain and Source when turned on.
It
is quite easy to buy MOSFETs that have very low RDS(on) ratings of less than
0.1
ohm. At 4 amps, this would mean that the voltage drop would be 0.4V per
MOSFET,
a definite improvement. The MOSFETs I chose had an RDS(on) rating of 0.04 ohms
which greatly improved my efficiency.

Now, when a MOSFET has a low RDS(on) rating, it usually has quite a high
current
rating typically in the 10s of amps. I needed 4 amps continous and the MOSFET I
chose offered 25 amps. Naturally, the lower the RDS(on) rating, the more
expensive the MOSFET. By the way, both N and P types of MOSFETs are available
in
TO220 packages. The cost is less than $5.00 for good MOSFET.

MOSFET manufacturers

Low RDS(on) P channel MOSFETs are more difficult to find than N channel. I had
to resign myself to a higher rated P channel MOSFET. There are quite a few
MOSFET manufacturers: MOTOROLA, International Rectifier, National Semiconductor
to name a few.

How a MOSFET works

MOSFETs work by applying a voltage to the Gate. The input Gate voltage controls
the output Drain current. They call this transconductance. When a positive
voltage greater than the Gate threshold voltage is applied, the MOSFET turns
on
(Q4 & Q6 - N Channel only in Figure 1). The P channel works in reverse
(See
Q3 & Q5) - negative device.

IMPORTANT:

MOSFETs are extremely static sensitive but more important is that if the
Gate is left open (no connection), the MOSFET can self- destruct. The Gate is a
very high impedance device (10+ megohms) and noise can trigger the MOSFET.
Resistors R3, R4, R6 & R8 have been added specifically to stop the MOSFET
from self destructing. It is very important to install these resistors FIRST
before installing the MOSFETs. You will find that after these resistors are
installed that the MOSFETs are quite stable devices. The resistors pull-down
the
Gates and turn off the MOSFETs, not to mention add some static protection.

Back EMF protection

D1 to D4 route back EMF from the motor back to the power supply. Some MOSFETs
(actually most) have these diodes built-in, so they may not be necessary.

Q1 & Q2 are NPN transistors that control the DC motor action.

STOP Mode

When A=0 and B=0, the motor is stopped. R3 and R4 pull up the Gates of Q3 and
Q5 respectively and turn off the MOSFETs.

REVERSE Mode

When A=0 and B=1 (+5V), the motor is in reverse. Q1 is turned off and Q3 is
turned off due to R3. Q2 is turned on by the voltage at B. Q2's collector
pulls
Q5's Gate to ground. This turns on Q5 (P channel needs more -ve voltage than
Source to turn on). The -ve side of the motor is raised to +12V. R5 raises
Q4's Gate to +11V or so which turns on Q4. Q4's Drain goes to ground which
makes
the +ve side of the motor go to ground. R7 is also connected to the +ve side
of
the motor which pulls down Q6's Gate and makes sure that it is turned off. The
current path for the motor is from +12V to Q5 to -ve contact to +ve contact to
Q4 to ground.

FORWARD Mode

When A=1 and B=0, the motor is in forward. Q2 is turned off and Q5 is turned
off due to R4. Q1 is turned on due to the voltage at A and Q1's collector goes
to ground. This turns on Q3 which raises the motor's +ve side to +12V. R7
raises Q6's Gate voltage and turns it on. When Q6 turns on, R5 makes sure that
Q4 remains off. The current path for the motor is from +12V to Q3 to +ve
contact to -ve contact to Q6 to ground.

NOT ALLOWED Mode (or fuse test
mode)

IF A=1 and B=1 then all MOSFETs turn on which shorts out the power supply
among other things - Not recommended.

Miscellaneous Info

The tricolor LED allows you to test the circuit without connecting the motor.
The LED will be green for one direction and red for the other. Handy test.

Motors make a lot of electrical noise from the brushes when running and
huge electrical spikes when stopping, starting and especially changing
direction. C1 and C2 try to suppress the noise spikes. Negative spikes are
shorted to either ground or the power supply by D1 to D4. Z1 tries to clip the
positive spikes.

Try to keep the motor supply separate from the logic supply if possible or
go to extreme filtering techniques using coils, diodes and capacitors to
filter
out the motor noise.